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Record proton-boron fusion rate achieved

Record p-11B fusion achieved; a good reason to celebrate with a bar of chocolate

In this purple target chamber, interestingly named Milka (and also colored as the Milka cow, see photo), a research team led by Christine Labaune at the Ecole Polytechnique in Palaiseau, France, fuse protons and boron-11 nuclei using a laser-accelerated proton beam and high-intensity laser pulse.

In the October-2013 edition of Nature Communications, they reported a new record fusion rate: an estimated 80 million fusion reactions during the 1.5 nanoseconds that the laser fired, which is at least 100 times more than any previous proton-boron experiment.

Experimental chamber in which the proton-boron fusion occured (Click to enlarge)

No harmful neutrons

One of the big challenges in fusion energy research is to cope with neutrons released from deuterium-tritium (D-T) reactions in a fusion reactor. Neutrons can make ordinary materials radioactive, and their energy is difficult to capture. As an alternative, the research team behind Milka focuses on fusion reactions between protons and boron-11 nuclei, which released alpha (helium) particles, but no harmful neutrons.

New laser technology put in action against the lower cross section

Proton-boron fusion reactions however require much higher temperature to ignite than D-T reactions designed for ITER and the National Ignition Facility and even at these higher temperatures the p-B cross section is lower. The methods of the D-T research facilities to heat and crush hydrogen in the hopes of creating a self-sustaining burn, can unfortunately not be directly applied when using proton-boron fuel.

Fortunately, the advance and rise of high-intensity pulse lasers technology makes it possible to adopt a different approach that creates extreme states of matter under conditions far from thermal equilibrium. This relaxes the equilibrium requirement and allows us to widen the range of isotopes that can be used as fusion fuels, e.g. to proton-boron.

The two-laser setup

The record was achieved by using a two-laser system (see image below). One laser ("Nano pulse" at the bottom in the image below) creates a short-lived boron plasma, by heating boron atoms from a boron sample. The other laser ("Pico pulse" on the left in the image below) is used to accelerate protons from an Aluminium target which then smash into the boron nuclei, fuse together and release Beryllium and Helium nuclei.

Experimental set-up: the laser beam configuration, the target arrangement and the diagnostics. [Source: Nature Communications]

Timing was crucial in the experiment, as the pulse of protons - lasting about one picosecond - must be precisely synchronized with the nano-second laser to slam into the boron target at the same time. In addition, the proton beam has to be preceded by a beam of electrons, to push away electrons in the boron plasma to increase the chance of the protons to collide with the boron nuclei and initiate fusion reactions.

Not a reactor yet, but still an important proof a principle

The scheme of the proton-boron setup does not give us a blueprint for a commercial reactor design yet, but it is still an important proof of principle. Considering the new method's early stage of development, the team of Labaune sees many opportunities for improvements, so that one day - aneutronic fusion might become the standard.

This new experimental approach may also provide opportunities to study other light nuclei, study 'early-universe reactions' of astrophysical interest, explore aneutronic fusion in dense plasmas and develop new insights in fast-iginition fusion schemes.

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